[OpenCL] Rename lang mode flag for C++ mode

Rename lang mode flag to -cl-std=clc++/-cl-std=CLC++
or -std=clc++/-std=CLC++.

This aligns with OpenCL C conversion and removes ambiguity
with OpenCL C++. 

Differential Revision: https://reviews.llvm.org/D65102

llvm-svn: 367008